This tutorial assumes that you know what CHDK is and have successfully identified your firmware version and loaded the correct CHDK build onto your cameras memory card (for the S100 see here and look out for the link to a quick step by step guide). I have also assumed that you will be able to work out how to scroll up and down the menus and select options by pressing FUNC SET. It is quite intuitive.
Different CHDK builds for this and other cameras may look slightly different but these instructions should still help you get you started.
Once you have set up your card lock it and put it back into the camera. Now switch it on by either pressing play or the on/off button. You should then see the CHDK splash screen below.
This will disappear and your camera will function normally. Press play and you will see alt at the bottom of the screen. CHDK is now running and capable of overriding the cameras settings.
CHDK allows you to set and control many parameters and if you loose track of what you have done and want to clear the settings go to CHDK Settings and scroll down to Reset Options to Default. With such a complex set of options this can be quicker and easier than trying to find a wrong setting that is causing the camera to misbehave.
To override the camera settings select Enhanced Photo Options in the main menu
Set Disable Overrides to NO and then set any options that you want. Here I have only selected Override Av (i.e aperture) and set the value to 2.05 which is the maximum setting that it will allow letting maximum light get to the sensor. Your preferences may be different. With the camera set in Tv mode the speed and ISO can be set as usual without going into CHDK.
Press menu to exit and then press FUNC SET to open the Script menu.
You will now see your selected script at the bottom of the screen and can set any variables. In this case the interval has been set to 2 seconds. Press back and then the shutter button to start the script. The script will run until you turn off the camera, or the card fills up, or the battery gives up.
This script will work for both KAP (kite aerial photography) and PAP (pole aerial photography).